The invention relates to air conditioning systems and particularly to a control apparatus for insuring that, at all times during the year, a predetermined minimum amount of fresh air is taken into the building in which the air conditioning system is located.
In order to insure comfortable working conditions or living conditions in large buildings or large building complexes, local codes generally require that a minimum amount of fresh air be supplied to the building during its occupancy. In some cases, particularly in the case of a variable volume system, it is difficult to maintain a predetermined flow of fresh air into the building.
A typical variable volume system will include a plurality of zones the temperature in each of which is to be controlled. Each zone is supplied with conditioned air through an associated variable damper the position of which is controlled by a thermostat located in its associated zone. Return air from each of the zones is supplied to a mixing chamber through a return air damper and is mixed with fresh air from a variable fresh air damper. The mixture of fresh air and return air, as well as the minimum fresh air supplied through a minimum fresh air damper, are temperature and humidity treated and supplied to the zones by a fan. The system also includes a static pressure sensor located in the output of the fan which senses the pressure of output and regulates the fan accordingly. Under certain conditions, it is possible for this system to fail to supply the minimum amount of fresh air provided for in the local codes.
These conditions may exist, for example, where the temperature outdoors is very cold and, therefore, the variable fresh air damper is closed. Assuming that the thermostat located in one of the zones calls for a reduction in the air supplied to the zone, the damper will begin to close which will increase the static output pressure of the fan. This increase in pressure is sensed by the static pressure sensor and is used to reduce the amount of air being discharged by the fan. This reduction will result in a reduction of the amount of return air being drawn into the system and a reduction in the amount of minimum fresh air supplied through the minimum fresh air damper.
Thus, it can be seen that an arrangement must be provided to insure that a minimum amount of fresh air is always supplied to the air conditioning system.